Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as portable communication devices (cellular telephones), personal digital assistants (PDAs) and other communication devices. An RF transmitter must transmit using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. In a typical global system for mobile communications (GSM) communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator.
In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used, thus allowing efficient transmission of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, such as that employed in IS-136, however, use a modulation scheme in which the transmitted signal is both phase modulated (PM) and amplitude modulated (AM). Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM transmitter hardware is not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the talk-time and standby time of the portable communication device on a battery charge.
This condition is further complicated because GSM transmitters transmit in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. In GSM this power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the control port of the power amplifier.
When attempting to include an AM component in a phase modulated GSM type modulation system, the power control loop will attenuate the amplitude variations present in the signal in an attempt to maintain a constant output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal.
In such systems in which transmit signals contain both PM and AM components, the output power can be controlled by applying a pre-determined control voltage to the power amplifier. Unfortunately, this requires the use of a power amplifier with a highly linear control characteristic and wide dynamic control range. In general the highly efficient power amplifiers used in GSM transmitters do not normally exhibit these properties to a sufficient degree. In non-burst transmission systems the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method to control the output power is to “pre-distort” the modulated signal in such a way that the power control loop will cancel the effect of the pre-distortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, these methods are costly, inefficient and require significant calibration time, dedicated test equipment sensors and fine control over the input power to the power amplifier.
Further, in those transmission standards in which the signal sent to a power amplifier contains both a PM and an AM component, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.
Further still, in transmission systems in which a combined AM and PM signal is used in a closed power control loop, it is difficult to obtain the full dynamic range in the AM signal to encompass all output power levels and to obtain sufficient dynamic range to smoothly control the ramp-up and ramp-down of the output power. In a closed power control loop system, it is important to maintain a constant, or nearly constant, loop parameters. In a system in which the AM is injected onto the transmit signal via the AM power control loop, the bandwidth requirements of the power control loop (both absolute value and consistency over the power range) are quite stringent. When a non-linear power amplifier is used to improve overall system efficiency, the gain control characteristics of the power amplifier tend to vary significantly over the range of operation and with temperature change of the power amplifier. This variation of the gain control characteristics impede system performance parameters, such as loop bandwidth, modulation accuracy, loop stability, variation in the power amplifier/system turn-on threshold (which can be thought of as a lowest reliable controlled output power), and other parameters. A significant portion of the variation of the gain control characteristics is due to the gain control function of the power amplifier (PA). Compensating the gain control characteristic of the PA by predistorting the transfer function of the PA gain control element is extremely difficult because the slope of the gain control signal could be as high as 150-250 dB/volt, especially at low power levels, thereby making predistortion of the transfer function of the gain control signal impractical.